294 V. C. Illing—The Search for Oil. 



may mean nothing or it may mean everything. It is only of 

 economic value when it is indicative of an underlying porous 

 reservoir rock impregnated with petroleum. 



A series of small shows of gas, oil, and asphalt, distributed 

 haphazard in a thick mass of marine or deltaic sediments, with no 

 consistency of geological horizons, and not associated with fault 

 planes, can hardly be significant of an underground oil reservoir. 

 The formation of hydrocarbons in small quantities is quite common 

 in nature ; it is the large accumulations which are more rare. If 

 these small oil and gas shows were the result of gradual upward 

 migration from a lower formation, it would be natural to expect 

 that the movement would largely take place along the easiest planes 

 of egress, i.e. faults, and porous strata. The extensive mining of the 

 Coal-Measures would have given ample opportunity to illustrate this 

 feature. Admittedly faults in softer strata tend to become sealed by 

 a plastering effect, but even so they form easier avenues of passage 

 than the undisturbed strata. The absence of any evidence of this 

 upward migration along fracture planes, and the promiscuous 

 distribution of the insignificant oil and gas shows, justify the 

 assumption that they are in the main of local origin. In view of the 

 fractured nature of the Carboniferous rocks, it is incredible that 

 large supplies of oil could be held in the lower portions of these 

 strata without giving abundant evidence of migration. The known 

 tendency of these materials to take advantage of every shatter-plane, 

 which is exemplified in all disturbed oilfields, is so characteristic 

 that it may be taken as a general law. 



But the question of the indications of oil is by no means exhausted. 

 It has been previously noted that in the present search for oil, 

 reliance is being placed largely on the possibility of oil occurring in 

 the porous Millstone Grits or Carboniferous Limestone, where these 

 are capped by impervious shales. If this assumption is true, it 

 ought to be found that wherever there exist anticlines composed of 

 these porous strata, wherein denudation is at present just stripping 

 off the overlying impervious beds and revealing the crest of the 

 underlying reservoir rock, the exposures of the latter ought to yield 

 unmistakable evidence of an oil impregnation,, even though most 

 of the lighter oil may have been volatilized. Two excellent examples 

 of such partially denuded domes occur at Ashover, and a few miles 

 north-east of Leek. Both of these have been examined by the 

 writer; neither of them yields evidence of oil impregnation beyond 

 the insignificant amount usual to the Carboniferous elsewhere. 



Thus it is seen that when the porous horizons of the Lower 

 Carboniferous rocks are exposed in the cores of domes, there is no 

 adequate oil impregnation or remains of such. Also, when these 

 porous horizons are covered by impervious shales, there is no 

 indication of oil migration such as would inevitably occur where the 

 numerous fault-planes in the Coal-measures crossed the position of 

 the underlying oil-pools. These two factors indicate that little or 

 no value can be attached to the so-called indications of petroleum 

 in Britain. These are merely insignificant examples of the 

 universality of bitumen in minute quantities. 



